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A case report of patients with melanoma on the feet suggests that mechanical stress may increase the formation of melanomas on the plantar surface.

Note that the plantar melanomas arose most often on the two areas of the feet where plantar pressure and shear stress exert the most mechanical pressure.

Could the mechanical stress of standing, walking, running, and other forces contribute to the development of a rare type of melanoma on the bottom of the feet?

After analyzing data on 123 cases accumulated over 25 years, Japanese investigators found the evidence suggestive but not conclusive. The plantar melanomas arose most often on the two areas of the feet where plantar pressure and shear stress exert the most mechanical pressure. The same types of stress are associated with formation of calluses and with diabetic ulceration of the feet.

"Taken together with these observations, our results suggest that mechanical stress also increases the formation of melanomas on the plantar surface," Ryuhei Okuyama, MD, PhD, of Shinshu University in Matsumoto, and colleagues concluded in a research letter published online in the New England Journal of Medicine. "Studies to determine the pathogenesis of melanoma in plantar regions are lacking."

Though intriguing, the evidence falls short of making a case for mechanical stress as a factor in plantar melanoma, said J. Leonard Lichtenfeld, MD, of the American Cancer Society.

"The study is interesting and represents a nice piece of research, going through the records and plotting the size and location of the melanomas, and they showed that the melanomas occurred most often in the areas subjected to the most pressure," Lichtenfeld told MedPage Today. "However, it's not sufficient to conclude that trauma itself is a causative factor. It may be a contributing factor but it may not be the only factor."

"We know a lot more about the typical melanoma, which is usually related to sun exposure," he added. "Through genetic analysis, we can now find a genetic signature of ultraviolet light in many melanomas that occur in sun-exposed parts of the body. We can determine whether there is any relationship to ultraviolet exposure. The authors made no comment, nor would I have expected them to do that kind of research on these lesions, and we don't know what the outcome would have been."

The findings also should be considered within the context of the rareness of plantar melanomas, said Lichtenfeld, noting the 25-year period required to accumulate 123 cases. Many clinicians and patients are unaccustomed to looking for melanoma and other skin cancers on the bottom of the feet, which would the relative thickness of the melanomas included in the study.

As Lichtenfeld noted, genetic analyses have shown that melanomas arising on sun-exposed areas of the body often have oncogenic BRAF mutations. In contrast, such mutations rarely occur in palmoplantar melanomas, Okuyama's group pointed out in the background information of their report. Other potential causative factors have received scant attention in melanoma research, providing a rationale to examine a possible role for mechanical stress.

The authors retrospectively reviewed data for 54 men and 69 women treated for plantar melanomas from January 1990 through December 2014. The patients had a mean age of 73.5. The melanoma arose on the sole of the right foot in 62 patients and the left sole in 61. Melanoma was confirmed by histopathology in all cases, and the authors excluded patients with subungual or periungual lesions.

The Breslow thickness of the melanomas was in situ in 28 patients, ≤1 mm in 12, 1.01 to 2.00 mm in 20, 2.01 to 4.00 mm in 28, and >4.00 mm in 33. The melanomas arose predominantly in the rear of the foot (n=50) and front (n=32), less often in the middle of the foot (n=14) and rarely in the arch (n=3). Lesion location did not correlate with Breslow thickness.

Trauma and other factors have been reported to increase the risk of melanoma arising in skin areas not typically exposed to sunlight, the authors noted. In the current study, "we found that conditions of the rear of the foot and front of the foot were more conducive to the development of melanomas than other areas of the plantar surface."

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